It has long been known in conjunction with absorption spectrometry to use electromagnetic waves, particularly light beams or bundles (groups) of light rays, that have a frequency in the infrared range, and to allow said beams or light bundles to pass along an optical measuring path extremity and to arrange for these light beams to be received by a detector.
In the case of gas sensors that are based on absorption technology, the light beams shall be allowed to pass through a gas sample enclosed in the cavity of a gas cell, in which a given volume of a chosen gas will result in frequency-dependent absorption of the light rays.
In this respect, a detector is adapted to detect the frequency spectrum of the light beams in respect of a gas or gas concentration currently in the cavity, and the gas or gas concentration can be determined by evaluating the intensity of the detected light beams in relation to a chosen intensity of the incident light beams and the absorption coefficient of the gas present in the cavity with respect to the light concerned, or the electromagnetic wavelength.
The cavity in the gas cell is normally formed by at least one first and one second part whose internal surfaces have each been treated to provide surfaces that will strongly reflect the incoming light beams.
This treatment will normally comprise coating said inner surface with one or more metal layers, wherewith the metal layer last applied forms the reflective surface.
The metal and the method chosen for this coating process will depend on the desired optical quality of the surfaces and for which optical wavelength or wavelengths the surfaces shall provide the reflection intended. The material in the body of the gas cell shall also be taken into consideration.
By way of example of the present standpoint of techniques can be mentioned the gas sensors illustrated and described in International Patent Application PCT/SE96/01448, Publication No. WO97/10460, and International Patent Application PCT/SE97/01366, Publication No. WO98/09152.
The contents of the following patent publications can also be considered to belong to the present state of the art as related to the technical field of the present invention.
D1: U.S. Pat. No. 5,170,064
This publication illustrates and describes a gas analyzer and in particular an analyzer for determining the instant, or current, concentrations of one or more gases.
The analyzer comprises a body that includes a cavity which includes an elliptically shaped light-reflective surface.
This surface is designed to define two focal points.
In this regard, there is provided a chamber (2) for enclosing the gas volume to be measured or assayed, said chamber (2) having a chosen height dimension, and a chamber (4) which is intended to enclose an inert gas and which has a height commensurate with half the height of the chamber (2).
There is also included a further chamber (3) which is used to enclose a reference gas, wherein the height of this further chamber is equal to the height of the chamber (4).
It is also proposed in this prior publication that the radiation source (24) shall be placed at a first focal point (11), while two radiation detectors (26, 27) shall be placed at a second focal point (12).
Each of the radiation detectors (26, 27) is allocated its respective chamber, where one chamber is intended for the inert gas and the other chamber is intended for the gas used as a reference.
The analyzer also includes partition walls (17, 18) or windows that form a gas-tight connection between an upper wall part (14a) and a lower wall part (14b), and also a partition wall (16) that functions to separate the chambers (3, 4) one from the other.
The chamber (2) includes holes (15) that allow the gas to be assayed or measured to pass through the chamber (2).
The partition walls (17, 18) are placed significantly closer to the focal points (12) than to the focal point (11), so as to create a measuring chamber (12) of relatively large measuring volume.
D2: U.S. Pat. No. 4,245,910
This publication illustrates and describes an arrangement adapted to function as a dust indicator which enables the presence in the gas of small particles to be detected, these particles either being in a solid or a liquid state.
In this case, concentrated, essentially parallel, light beams (1) of chosen wavelength are directed through the measuring chamber (2) that encloses the gas to be analyzed.
The light beams are enclosed by an inwardly directed, reflecting elliptical and cylindrical reflector (4) such that the light beams will coincide with a focal axis (B1).
A light-sensitive detector (6) is disposed at the other focal axis (B2) of said reflector.
The arrangement also includes a part (7) which has been designed to allow the gas being analyzed to pass into the measuring chamber, wherewith said part can be given any desired form, and wherein the detector (6) can be disposed in an opening in said wall part.
It is proposed especially that the inner surface of said part (7) shall be non-reflective.